I am new to galaxy and its tool shed. I have been looking for genome assemblers and came across a tutorial for NGS velvet assembler that should help in de novo assembly. But I can't see this tool in the toolshed. Please suggest how a de novo can be run on the server as I haven't installed galaxy on my PC.

I guess you have checked the main toolshed. However there is another one which has a lot of de Novo tools not currently on main. It is run by the journal Gigascience and features tools developed by the BGI.